A strike is an organized and collective work stoppage by a group of employees, usually with the goal of forcing the employer to accept the employees’ demands, or in protest of an alleged unfair labor practice. A union might call a strike to put pressure on the employer to accept the union’s demands on wages, benefits, or other terms and conditions of employment. The striking employees might organize picket lines in front of their work areas (and often elsewhere) where they hold signs, march, and chant.
Since a strike is about withholding labor, an employee is not formally on strike if they participate in protests or demonstrations but are otherwise performing their work obligations. Additional information from the National Labor Relations Board (NLRB) regarding the right to strike, as well as lawful and unlawful strikes can be found here.